The invention relates to combustion engine carburetors having an automatic choke system or starting system.
There exists starting systems for carburetors having an intake pipe, a main operator-controlled throttle element, a main fuel jet system, a choke valve located upstream of the throttle element and biased towards closure by a thermostat element and biased towards opening by the air flow in said pipe, a pneumatic element constructed to bias said choke valve towards opening, auxiliary means for adjustment of the minimum opening of the main throttle element in dependence upon engine temperature. The pneumatic element communicates via duct means with the downstream side of the main throttle element so that the depression on the downstream side of the main throttle element controls the pneumatic element. The minimum-opening position of the main throttle element can be determined by a fast-idling cam (French Patent Specification No. 1,302,536).
In starting systems of this kind, the underpressure or depression which is operative at the main jet system where the thermostat element is keeping the eccentric flap or valve closed is a means of ensuring a fuel-air mixture of increased richness while the engine starter is being operated. The richness should be decreased without delay once the engine fires, otherwise it will choke and stall. Rapid decrease in richness is produced by opening the eccentric valve to some extent responsive to the depression downstream of the main throttle valve acting on the pneumatic element controlling the choke valve or flap.
There are, however, disadvantages in controlling the opening of the air valve by a pneumatic element since the system has to meet requirements which to some extent are conflicting. The thermostat element which tends to close the air valve produces a closing torque which is higher in proportion as the engine temperature at starting is lower; consequently, the pneumatic element starts to open the air valve only when the depression downstream of the main throttle element exceeds a valve which is higher in proportion as engine temperature is lower, since the opposing torque produced by the thermostat element increases as temperature decreases.
In other words, the engine speed at which the air valve starts to open as engine speed increases is high (strong depression in the inlet tube) at very low starting temperatures (for instance approximately - 20.degree. C) and is low (slight depression in the inlet tube) for higher starting temperatures, for instance of approximately 20.degree. C.
In the latter case, the air valve opens too fast and the engine once started may stall. For overcoming that drawback and ensuring satisfactory operation at starting temperatures of the order of 20.degree. C, the opening movement of the choke or flat may be delayed. Then difficulties arise at low temperatures (- 20.degree. C) due to the air valve opening too slowly and flooding the engine.